The present invention relates to a supercharged internal combustion engine having a common exhaust manifold and a common combustion air manifold for all combustion chambers. The engine also has a plurality of exhaust-driven superchargers that are staggered as a function of the output of the internal combustion engine, wherein the superchargers are engageable or disengageable with the exhaust manifold via their exhaust-driven turbine. The engine also has a charging fan that is driven by a separate motor and that is disposed upstream and in series with the compressor of one of the superchargers.
With the aid of the supercharging, it is possible to realize in a small internal combustion engine the output of an otherwise considerably larger internal combustion engine. In the case of a prescribed rated output, the engine can be considerably smaller due to its supercharging. During the supercharging of internal combustion engines having exhaust-driven superchargers, there results the conflicting objectives that on the one hand due to the supercharging an increase of the rated output is possible, and on the other hand for this purpose the geometrical/mechanical compression must be proportionally reduced relative to the desired increase in rated output. Due to the reduction of the geometrical/mechanical compression, however, the output or torque of the supercharged internal combustion engine drops in an over-proportional manner in the lower speed range, which is attributable to the operating speed plot of the output curve, which for exhaust-driven superchargers is very steep. A fluid-producing mechanism can bring about an optimal flow, and hence a high output, only within a very narrow speed range. This is the opposite of the desire in internal combustion engines of land vehicles for the production of a propulsion hyperbola. However, by the use of a plurality of exhaust-driven superchargers, not only the operating speed range of the internal combustion engine, but also the special volumetric operating size of the exhaust-driven superchargers, can be divided. However, the resulting, chronologically offset engagement of the superchargers easily leads the supercharging system into the range of the pumping and to the collapse of the combustion air flow in the compressor that is to be engaged if, for example, the air, which initially flows from the second compressor at very low pressure via a discharge valve into the atmosphere, is intended to pass into the combustion air manifold of the internal combustion engine that is already under the full pressure of the first compressor, in other words from a flow-producing mechanism having a relatively high flow velocity and little possibility for building up pressure to a reciprocating engine having relative low flow velocity and a relatively high possibility for building up pressure.
Mechanical positive-displacement chargers, which can be driven by the internal combustion engine itself or by a separate motor, easily convey the air required for idling of the internal combustion engine, but in the high output/high speed range of the internal combustion engine cannot compete with an exhaust-driven supercharger. Therefore, it is customary on an internal combustion engine to combine the respective advantages of an exhaust-driven supercharger and a positive-displacement compressor by a series connection or a parallel connection of the two chargers. This is particularly applicable for counteracting the so-called turbo hole or leak, which results when upon start-up of the internal combustion engine, a single or first exhaust-driven supercharger comes only slowly up to speed due to the slowly increasing generation of exhaust gas.
A supercharged internal combustion engine of the aforementioned type is known from the publication DE 40 40 939 C1. With this internal combustion engine, as a separate motor for the preliminary blower, and as the main motor, a respective rotary piston diesel engine having a low compression is used, with greatly unequal output dimensioning for the main motor and the smaller preliminary blower motor; for the drive of the preliminary blower, an electrical starter motor is additionally provided. From an output signal of the main motor, at least one two-stage exhaust-driven turbo charger is connectable parallel to the series connection of the preliminary blower and of the charging blower that is driven by the exhaust-driven turbine. In this way, the preliminary blower motor can be reduced in size in conformity with its proportional contribution to the charge output. With a connectability of merely one two-stage exhaust-driven turbo charger, the necessary size of the preliminary blower motor can be cut in half, and with the staggered connectability of two two-stage exhaust-driven turbo chargers, the necessary size of the preliminary blower motor can be one third. The so-called pumping during the chronologically offset additional connection of exhaust-driven turbo chargers is not addressed. However, a loss of compressed air from the region of the combustion air manifold due to a backward flowing of combustion air in each of the exhaust-driven turbo chargers that is to be connected or engaged is to be avoided. This is particularly applicable to diesel engines having a pressure threshold that is critical for its spontaneous ignition.
It is an object of the present invention, for a supercharged internal combustion engine, in conjunction with an increase of output, to provide an improved acceleration characteristic in the starting range, and a transition-harmonic operating spectrum from very low speed at partial load and full throttle up to high speed at partial load and full throttle, and to enable a staggered engagement and disengagement of exhaust-driven superchargers without the aforementioned pump effect.